The present invention relates to an image forming device and particularly to such a device that includes a plurality of recording heads.
An ink-jet printer, a type of ink-jet image forming devices, is a device employing an ink-jet recording method which uses a head with aligned nozzles to eject ink drops therefrom to perform printing, with characteristics such as low noise, space-saving, etc. The head is fixedly mounted in position on a carriage, which is scanned across a recording paper sheet while ink drops being ejected to form an image on the paper sheet. The carriage also carries an interface circuit board necessary for driving the head.
Each nozzle is always filled with ink and provided therein with a heater element, which is heated with driving pulses to generate air bubbles in the nozzle. The bubbles are swelled to eject part of the ink within the nozzle outwardly so as to create an ink drop which will land on the paper sheet to make a printed dot.
In such an image forming device, one recording head is formed with a multitude of aligned nozzles, and hence, the nozzles in the same head are filled with the same ink from the same ink tank. To form a full-color image by an ink-jet printer, three colors, i. e., cyan, magenta and yellow, of ink are used so as to superimpose them to form an image, which realizes any color on the recording paper sheet. Therefore, a full-color ink-jet printer requires at least three heads. In practice, however, one more head of black is provided since beautiful black is not obtained with the above three colors. Thus, four heads in total, i.e., black (K), cyan (C), magenta (M) and yellow (Y), are used for printing.
The heads are located on a carriage such that the nozzles of each head are aligned in the recording paper travelling direction (sub-scanning direction). While the carriage is being scanned in the direction (main-scanning direction) perpendicular to the sub-scanning direction, ink drops are ejected onto the recording paper from the nozzles according to print data. One scanning of the carriage over a print region results in the printing of a band of image. Then, the recording paper sheet is transported in the sub-scanning direction by a predetermined amount to perform the printing of a second band, in the same manner as with the first band. By repeating such operations for third, fourth, . . . bands, an image is completed.
The four heads are removably mounted on the carriage, and hence, what determines a print position (i. e., a head position) is the carriage. In general, a positional sensor is provided in position on the carriage to detect the position of the carriage in a unit of dot in cooperation with a linear scale which is disposed along the scanning direction of the carriage. The output of the positional sensor creates the timing of the printing. In an ink-jet printer with a plurality of such heads for printing in both, back and forth, scanning directions, deviation in the position of heads mounted by a user, difference in characteristics of the heads themselves, and change in speed of the carriage produce deviation in the printing position of each head, resulting in an offset (registration offset) of the printed image for each head. For example, printed rule lines offset or vertical or horizontal stripes appear in the image, which sometimes affects significantly the image quality. The registration offset includes a sub-scanning directional offset (vertical registration offset) and a main-scanning directional offset (horizontal registration offset), for each of which another registration offset could occur between the back and forth scanning directions.
Once such registration offsets are detected, the influence of the offsets will be eliminated by correcting the printing such that with respect to the vertical registration offset, the group of nozzles to be actually used for printing is shifted in the sub-scanning direction (where the number of the nozzles of a head is set greater than that of actually used at a time), and with respect to the horizontal registration offset, the timing of ejecting ink is adjusted earlier or later.
In order to perform such correction, however, it is required for the device to accurately recognize the extent of the registration offset. The measuring of the registration offset generally falls within either of the following two methods. One is a manual registration adjustment wherein the device prints a pattern by which a user readily recognize the registration offset with the unaided eye based on the printed pattern and the user manually inputs the amount of the recognized registration offset to the device. Another is an automatic registration adjustment in which the device automatically detects the offset.
Further, the automatic registration adjustment includes a method in which the device prints a predetermined test pattern for an optical sensor to detect the pattern to determine the amount of the registration offset, and another which uses an optical ink drop detector which detects the position of an ink drop ejected from the head so that a calculation is made so as to obtain the position where the ink drop hits the medium.
Among the two automatic registration adjustment methods, the present invention employs the one that uses the test pattern to detect the amount of the registration offset. This basic technique is disclosed in Japanese patent application laid-open (KOKAI) No. 7-323582 which was filed by the applicant of this application. In this technique, a pattern-reading scan is made twice, firstly for detecting a center dot position of each of two given pattern elements, and secondly for measuring the interval of the detected two center dots. Also, an international patent publication No. WO097/14563 discloses a technique wherein two light receiving elements are employed to use the differential output thereof so as to improve the accuracy of the test pattern detection.
It is an object of the invention to provide an ink-jet image forming device capable of improving the accuracy of the test pattern detection with a single light receiving element.
It is another object of the invention to provide an ink-jet image forming device capable of reducing the time necessary for the registration adjustment by measuring the interval of the center dots of two given pattern elements in a single pattern-reading scan.
According to the present invention, there is provided an ink-jet image forming device in which a plurality of heads are scanned in a direction substantially perpendicular to a recording-medium travelling direction, the ink-jet image forming device, comprising: a test pattern printing means for printing a test pattern on a recording medium by using the plurality of heads; a light-reflection type optical sensor which is scanned across the test pattern printed on the recording medium for sequentially detecting pattern elements thereof; a binarizing circuit for binarizing an output of the light-reflection type optical sensor; a calculating circuit for obtaining a plurality of data items concerning intervals between a reference head of the plurality of heads and the other heads, according to the output of the binarizing circuit; and means for determining amounts of offsets in print position of the other heads relative to the reference head; the binarizing circuit, comprising: a peak-hold circuit for following slow change in the output of the light-reflection type optical sensor; a voltage-dividing circuit for dividing a held output of the peak-hold circuit; and a comparator for converting the output of the optical sensor into a bi-level signal by using the divided output of the voltage dividing circuit as a threshold.
With such an arrangement, the sensor output responsive to the test pattern is binarized with the threshold that dynamically changes according to the output of the peak-hold circuit which follows the slow change in the sensor output. Therefore, an adequate binarization is realized even with the deviation of the sensor output due to various factors. As a result, the offset in the inter-head printing position can accurately be detected so as to cancel the offset.
A sample-hold circuit may be used in place of the peak-hold circuit, the sample-hold circuit serving to sample and hold the output of the optical sensor during a predetermined period corresponding to the test pattern.
According to the present invention, there is provided another ink-jet image forming device in which a plurality of recording heads are scanned in a direction substantially perpendicular to a recording-medium travelling direction, the ink-jet image forming device, comprising: a test pattern printing means for printing a test pattern on a recording medium by using the plurality of heads; a light-reflection type optical sensor which is scanned across the test pattern printed on the recording medium for sequentially detecting pattern elements thereof; a binarizing circuit for binarizing an output of the light-reflection type optical sensor; a calculating circuit for obtaining a plurality of data items concerning intervals between a reference head of the plurality of heads and the other heads, according to the output of the binarizing circuit; and means for determining amounts of offsets in print position of the other heads relative to the reference head; the test pattern including a reference pattern element printed with the reference head, and a plurality of comparative pattern elements respectively printed with the plurality of heads at positions a constant designated interval away from the reference pattern element; and the calculating circuit obtaining an interval data item of one of the other heads relative to the reference head, based on the output of the optical sensor in a single scan across the test pattern.
Thus, the number of sensor scans for detecting the test pattern is reduced to one half, thereby reducing the time required for the process of correcting the offset of the head printing position.
More specifically, the calculating circuit may include a rising edge detector responsive to the output of the binarizing circuit for generating a pulse when detecting a rising edge thereof; a falling edge detector responsive to the output of the binarizing circuit for generating a pulse when detecting a falling edge thereof; a first counter for starting clock counting in response to a detected output of the rising edge detector; a register for holding a count value of the first counter in response to the pulse of the falling edge detector; a divider for halving the value held in the register; a first comparator for comparing the count value of the first counter with a predetermined set value so as to stop the counting operation of the first counter when both the values coincide with each other; a second counter for starting clock counting in response to the coincident output of the first comparator; a second comparator for comparing the count value of the second counter with the output of the divider so as to stop the counting operation of the second counter when both the values coincide; and a third counter for starting clock counting in response to the coincident output of the second comparator; the third counter stopping the clock counting in response to another coincident output of the second comparator which occurs again after staring the clock counting, the count of the third counter being used as the interval data item between the reference pattern element and the comparative pattern element.
Alternatively, the calculating circuit may include a rising edge detector responsive to the output of the binarizing circuit for generating a pulse when detecting a rising edge thereof; a falling edge detector responsive to the output of the binarizing circuit for generating a pulse when detecting a falling edge thereof; a first flip-flop for inverting an output thereof each time the pulse from the rising edge detector is received; a first counter responsive to the output of the first flip-flop for performing clock counting during a scanning period of the sensor from the leading-edge position of the reference pattern element to that of the comparative pattern element; a second flip-flop for inverting an output thereof each time an input pulse is received; a second counter responsive to the output of the second flip-flop for performing clock courting during a scanning period corresponding to a width of the reference pattern element; a third flip-flop for inverting an output thereof each time an input pulse is received; a third counter responsive to the output of the third flip-flop for performing clock counting during a scanning period corresponding to a width of the comparative pattern element; and a switching means responsive to a bi-level output of the binarizing circuit for supplying alternately to the second and third flip-flops a pair of two detected pulses of the rising and falling edge detectors; the determining means for obtaining an interval data item between the reference pattern element and the comparative pattern element on the basis of output values of the first, second and third counters, and for determining amounts of offset in print position of the other heads relative to the reference head.
In the latter calculating circuit, the test pattern may include a plurality of sets of pattern elements disposed in the sensor scanning direction, each of the sets including a reference pattern element and a plurality of comparative pattern elements; and wherein the first, second and third counters of the calculation circuit may accumulatively hold the respective count values during one scanning period of the sensor.